Methods and apparatus for measuring wave propagation time



D. C. ERDMAN METHODS AND APPARATUS FOR MEASURING April 5, 1960 WAVE PROPAGATION TIME '7 Sheets-Sheet 1 Filed July 18. 1955 .uk wha Q kl INVENTOR. DONALD C. ERDMN 'Arron/VHS,

Aprli 5, 1960 D. c. ERDMAN METHODS AND APPARATUS FOR MEASURING WAVE PROPAGATION TIME 7 Sheets-Sheet 2 Filed July 18, 1955 QOLO s Sugo N NN 33:/ INVENTOR.

LJIDONALD C. ERDMAN BY Me M,

ATTORNEYS Aprll 5, 1960 D. c. ERDMAN METHODS AND APPARATUS FOR MEASURING WAVE PROPAGATION TIME 7 Sheets-Sheet 3 Filed July 1B, 1955 Ow X www Aww

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A wm April 5, 1960 D. C. ERDMAN METHODS AND APPARATUS FOR MEASURING WAVE PROPAGATION TIME Filed July 1s, 1955 7 Sheets-Sheet 4 INVENTOR. DONALD C. ERDMN mi M A TTORNEYS April 5, 1960 D. c. ERDMAN METHODS AND APPARATUS FOR MEASURING WAVE PROPAGATION TIME 7 Sheets-Sheet 5 Filed July 18, 1955 Apnl 5, 1960 D. c. ERDMAN METHODS AND APPARATUS FOR MEASURING WAVE PROPAGATION TIME v 7 Sheets-Sheet 6 Filed July 18 1955 n ,fr/JMA? M TTRNEYS D. C. ERDMAN METHODS AND APPARATUS FOR MEASURING 7 Sheets-Sheet 7 Filed July 18.. 1955 Donald C. Erdman, Pasadena, Calif., assigner to Electrocircuits Incorporated, Pasadena, Calif., a corporation of California Appiicatinn July 1s, 195s, serial No. 522,542

9 claims. (cl. S24-68)' This invention relates to improvements in methods and apparatus for measuring the propagation time of waves along a path by the use of frequency or phase-modulated waves. The invention has particular reference to ultrasonic waves such as are employed in the non-destructive testing of materials `to detect hidden flaws; however, the invention is equallyapplicable to other arrangements in which the propagation time of waves is measured, such as in radar and sonar.

In order to measure the location of a target by the use of ultrasonic or radio frequency energy, the propagation time of waves from a source of energy tothe Y target and backto the source is=usually measured to provide an indication of the distance of the target from the source of energy. Usually the signals are pulse or frequencymodulated so that the reiiected signals can be Vdistinguished from one another and from the signals Y of the pulses.

Frequency-modulated arrangements require relatively less power becauseV they have a large dutyV cycle. In such arrangements,` the frequency of the signal which is produced by the source of energy is varied or swept periodically'lover a certain. frequency range.` The reected energy sweeps through the same frequency range as the original signal,` but the instantaneous frequency of the reflected energy is different from the instantaneous frequency of the signal then being produced by the source because ofthe time required for the reflected travel to and from the target. l

Ordinarily the reflected signal which is received is mixed with the frequency-modulated signal which is then being producedV by the source to provide a heterodyne or'beat note signal having a frequency which is equal tothe difference between the instantaneous frequencies of the received signal and the signal which is 'then being produced by the sourcel' The frequency of the heterodyne signal isgdetermined by the" propagation time f the reected signal, and hence it provides a measure of the distance :from Vthe source of energy to the target. The distance is ordinarily measured by using a frequency meter to measure the frequency of the heterodynesignal.

It is diicult to use such arrangements to detect multiple targets because precise control of the linearity of the frequency modulation is required in order to obtain good range resolution.

This diiculty is overcome in the present invention by providing two waves in time sequence having frequencies which varyor sweep periodically in substantially the same manner,with.the soundwave starting at' a later time and being oifsetfin frequency from that of the first wave. One or both ofthe waves may be transmitted nited States Patent ICC from thesource to the reflecting target, and the reflected waves are mixed with one or both of the waves which are then being produced by the source. An indicator arrangement provides an indication of the occurrence of heterodyne signals having a certain frequency.

The propagation time of the wave may be measured 1)l by adjusting the time at which the offset in frequency occurs between two waves, (2) by adjusting the amount of 'the frequency oiset between the two waves, (3) yby adjusting the'distance between the source of the waves and the target until the heterodyne signals have the frequency at which the indicator arrangement responds, (4) by the use of tunable or multiple filters, or (5) by any combination of items (l), (2), (3), and (4).

Preferably the parameter which is adjusted is varied periodically and a cathode-ray tube is employed in thev indicator with the trace of the cathode-ray tube being swept in synchronism with the periodical variations of invention;

Fig. 2 illustrates the waveforms of the signals which are produced inthe apparatus of Fig. 1;

Fig. 3 is a diagram illustrating a modication of the apparatus ofFig. l;

Fig. 4 is a circuit diagram showing another embodiment of the invention; i

Fig. 5 illustrates the wave form of signals which are produced in the apparatus of Fig. 4;

Figs. 6 and 7 are circuit diagrams showing modifications of theY apparatus of Fig.`4;

`Figs. 8 4and 9 arecircuit diagrams showing further embodiments ofthe invention; and v Fig.' 10 illustrates the wave form of signals which are produced `in the apparatus of Fig. 9.

Fig. l shows an embodiment of the invention in which two frequency-modulated Waves are transmitted in time sequence to the target, and both of the waves are em-` ployedf to provideqthe reflected signals and the signals for mixing with the reflected signals to produce heterodyne signals. The second wave starts at a later time and itis offset in frequency from the rst wave. The time at which the offset in frequency occurs is varied inV order tomeasure the propagation time of the waves. Av timing pulse generatorV 12 provides a series of equally spaced pulses A (see Fig. 2) which control the sequence of operations in the apparatus. The timing pulse Ygenerator is a variable repetition rate type, and the rate or periodicity of the pulses is controlled bythe voltage which is produced by a variable voltage source 14. The'variable'voltage source 14 may comprise a potentiometer 16 which is connected across a source of potential 18 and which is' driven by a motor 20, as illustrated in Fig. 1, or it maybe a conventional electronic sweep generator which provides periodic voltage excursions.

The timing pulses A are applied to a sweep generator 22 and to a bi-stable circuit 24. The sweep generator produces voltage excursions B having sawtooth wave form in synchronism with the timing pulses A. The bi-stable circuit produces signals C of rectangular wave form in synchronism Withthe voltage excursions B.

` VThe sawtooth signals B and the rectangular signals C are applied to a gated adder circuit 26 where they are summed to provide an output signal D of sawtooth wave form lwherein the adjacent voltage excursions'start from' .5a-sagem The output signal is applied to a reactanee l' 36. The output f the reac'tance tube'cireuit Ais applied to an oscillator 38, and itv serves to causethe o s'cillatorjto produce two waves which occur in time 'sequencej'and which have frequencies which vary lor sweep periodically Vin the same manner in synehronismwi'th heveltageiexcursions of Vthe signals D which are .produeedmbyxthe addition circuit. The 'second wave (in time). isdfffsetn'in frequency from 'the first wave an amountl which is proportional to the offset in Athe voltage -exc'nursionsD.

The output signals E 'of Ythe oscillator `are fappliejl through a power amplifier '40 and ahybrid net 'w'rnki12-to a crystal transducer 44 which "serves to ytrf'ilns'm'it'antifeceive ultrasonic signals. In order t illustrate ljth'e invention, the transducer `44 and a target 'are rsheyvn ijn-| mersed in a liquid 48. Y The liquid serves to conveyfthe ultrasonic waves with Arri'uch greater eliiciency than would be possible'in'air.y Y 7..; Y* V Y): ,j

A balancing network St) vis eonnectdv toftlehybrid network. The hybrid netwerk its balancing network are adjusted so that substantially all the ultrasonic energy which is 'producediat the loutputof the power amplifier is applied through the hybrid "network to "the transducer 44, and s'otha't substantially none of this signal is fed to the amplifier SZand-the mixing arrangement which follows it. The'hybrid Tntworkand its balancing n etworkware arranged'to nv ym bsta'ritially Vall `bf the Jreflected energy which is 'received by Ythe;'trarisnlilcer .44 to the lamplifier 52. The hybrid networkfm'ay beof any conventional type such las the 'antisidetonen/circuit"used in telephony. In radar arrangementswhieh'employ ultrahigh-fr'equency energy, the hybrid network may be a magic T circuit. A A

The echo signals which Iare'received by the transducer 44 are applied through the hybrid network 42 vand the amplifier 52 to 'a mixer A54. The 'output of the oscillator 38 is also applied to the mixer 54 so vthat thefreceived waves are mixed with the waves whichfare Vthen being produced to provide heterdyne signals.

The output of the mixer is `applieini 'througha bandpass or narrow-band tilterf'56 to aldete'c'tor 58. The filte'r 56 `may be arranged to pssheterdyne "signals: having any desired frequency. By iway of example, fh'e'flterfSlti may be tuned to pas`s`sg11`als vhaving affrequeny of 455 kilocycles. Such filters are'readily available becaiisethey are conventional types. l p l The detected signal -is amplified by a video amplifier 60 and applied to o'ne 'set of the 'deflection Aplates ofl a cathode-ray tube A62. j The variable voltagewliich isueproduced by the source 14 vis applied tothe other set 'of the deflection plates of thexcat'h'ode-'ray tube. Such adeiiection arrangement provides a'n A-type fscanfso 'thatithe location of the detected signlf'alori'g the horizontal 'axis of the cathode-ray tube provides a measure-ofthe distance to the target. s j Y The distance to the t'ar'get'isalso `lindicated by the time between the pulses A'which are lproduced by the 'timing pulse generator 'atltlie instant when@ signalfappears en the screen vof the'catho'de-'ray tube. lf desired, 5the signal which is producedjby the variable voltagefLsoureejM'may be adjusted manually andthedistanceto thetargetmay be ascertained by observing then'manual setting of fthe.

voltage sour'ce when@ signal is (produced on thefscreen of the cathde-r'ay tube. fnowver, ir is preferable that the signal which is 'pmdtid bythe jsnrq'e Vv1'4 befv'aied cyclically, as indicated in Fig. 1,-so tht'pluralreflecting surfaces may be indicated during each lcycle of operation.

In the apparatus of Fig. 1, 'eachz'tinie fthatja `reflected signal is received at the saine tiniethat vthe offset in 'freq 4 quency between the two frequency-modulated waves` occurs, a heterodyne signal having a beat frequency equal to that to which the filter 56 is tuned may be produced as illustrated by the wave forms E of Fig. 2. The time ta at which the echo signals occur is equal to the time t, at which the offset in frequency between the two waves occurs. In the example shown-in Figs. 1 and 2, the beat note will 'have a frequencyof 455 kilocycles,y as rillustrated by vthe shadedportion between `the echo signals and the signals E. This beat note will be .passed by the filter 56 to provide van indication on the screen of the cathode-ray tube. i V

The wave form E of Fig. 2 illustrates the Jcondition of operation wherein the time te at which th'e echo occurs does not coincide with the time tj at which the offset in frequency between 'the two waves occurs. For this condition, the heterodyne signals have a frequency which is different from that to which the filter 56 is tuned, and hencegno indication will be .provided lon the-screen of the cathode-ray tube. `'Instead of controlling the time tj 4at which the offset in frequency between the two lfrequency-modulated waves occurs, the amount of-ol'rsetin 'frequency between the` two waves may be adjusted in order `to produce heterodyne tion, a -timing pulse generator 12 which operates at a fixed rate is employedtorcontrol -the sequence of operations.v The amount of offset in frequency between the two waves isadjusted by apotentiometer 64 which-serves to controlvthevamplitude of the rectangular waves which are applied to the. addition circuit '26. The 'potentiometer 64-may be varied cyclically by the motor v'20, ask illusy trated in Fig. 3, or it may be adjusted manually if desired. Preferably; the 4potentiometer 64fis varied in synchronism with the signals which deflect` the beam'of the cathode-ray tube. This may b e achieved by'synchronizing the operation of the potentiometers 16 and '64, as illustrated-in Fig. 3.

iFig. 4 illustrates an alternative embodimentof the invention in which the time at which theofi'setein frequency between the two 'frequency-modulated waves occurs is varied. ,A This-embodimentofathe -inventionis similar to that of Fig. l, except that :the :hybrid Y network, fthe Vbalancing-network, and the mixer'circuitjare omitted.. A variablefgain power amplifier 66 is 'coupled between lthe oscillator 38 `and the transducer '44, vand the gain of the power amplifier Tis controlled in laccordance with the signalsC which are produced by the Vbi-stable circuit 24.

As illustrated inthe wave forms F of Fig. 5, the amplitude of the signal which is conveyed through th'e Vpower amplifier-6 6 is reduced duringalternate cycles of operation, Vand the wave of reduced amplitude is mixed with theechosignals in the amplifier 52m-provide heterodyne signals. -The amplifier 52-in`this .embodimentof the invention should Vbe Va Ytype which will not block when signals of large Yamplitude arerapplied "to it.

The operation 'of the apparatus of Fig. 4 is Aillustrated in-Fig. 5. When the time te yat which the -echo'occurs is equal Yto the time t, at vwhich the folset Vin lfrequency between the twofrequency-modulated waves'occurs, a'heter-` odyne signal will be produced-having a beat 'frequency which will pass through the filter-56 so as to cause anindication-on the screen 'of the 'cathode-ray vtube 62. The

shaded areas extending between thefechosigrialsand the signals Gvrepresent Ythe heterodyne signals which areproduced-forthis condition ofoperation.

Fig. -illustrates how the apparatus-of Pig. A4 -may'be modifiedso as to vary 'the amount-of Offset in frequency.

between the two frequency-modulated waves instead of the time at which the offsetin-frequency Ebetween `thetwo g waves occurs. This modification of the inventionv is similar to that discussed above with reference to Fig. 3 in that the amplitude of the rectangular waves C is varied in order to vary the amount of offset in frequency.

Fig. 7 illustrates how the apparatusof Fig. 4 may be modified so as to vary both the time at which the offset in frequency of the'two frequency-modulated waves occurs and the amount of offset in frequency between the two waves. In this embodiment of the invention, the time at which the offset in frequency'occurs is controlled by the setting of the potentiometer 16, and the amount of offset in frequency between the two waves is controlled by the potentiometer 64.

Fig. 8 illustrates an embodiment of they invention in which the time at which the offset in frequency between the two frequency-modulated waves and the amount of offset in frequency between the two waves is maintained constant, and the location of the transducer is adjusted to vary the length ofthe path along which the waves are propagated.

In this embodiment of the invention, the location of the transducer 44 is controlled bya motor 70, and the time at which the echo signals `are received may be controlled in this manner. A motor control circuit 71 causes the motor to run first in one direction and then in the opposite ldirection for predetermined periods of'time so as to move the transducer 44 up and'down periodically. The sweep of the trace of the cathode-ray tube 62 is controlled by a potentiometer 16 which is also actuated by the motor 70 so that the trace of the cathode-ray tube is moved in synchronism with movement of the transducer 44."` t

echo signal which occurs'atthe same time that the offset* in frequency occurs betweenthe two frequency-modulated waves which are produced by ythe-oscillator 38, a heterodyne signal vhaving a frequency which will be passed by the filter 56 is produced. Thissignal is detected and applied through the video amplifier 60 to produce an indication on the screen of the cathode-ray tube at a location which corresponds to the location of the portion of the target 46 that produced the echol signal.

It will be apparent that variation ,in` the length of the propagation path is all that is required to practice the technique which is illustrated in Fig. 8. Thismay be achieved by moving the target as well as by moving the transducer.

Although the apparatus of Figs. l to 8 has been discussed with particular reference to use at ultrasonic frequencies, it will be apparent that these embodiments of the invention may be employed at radio frequencies if desired. Fig. 9 illustrates one way in which the techniques of the present invention may be employed in a radar system.

In this embodiment of the invention, a variable rate timing pulse lgenerator 80 provides separate trains of pulses H and I for controlling the sequence of operations. The pulses H are applied to a sweep generator 82 which produces voltage excursions I having sawtooth wave form. The signals I are applied to a reactance tube circuit 84, and the output of the reactance tube circiut is employed to control the frequency of the signals which are produced by an oscillator 86. The output signals M of the oscillator 86 are applied Ato a radio transmitter 88 which transmits the radio frequency waves from an antenna 9G.

The reliected waves `are received by an antenna 92 and applied through a radio frequency amplifier 94 to a mixer 54.

The pulse signals I are applied to another sweep generator 95 which produces voltage excursions K which start at a higher level than the voltage excursions I. The signals K are applied to a reactance tube circuit 96 which controls the frequency of the signals which are produced by an oscillator 98. The oscillator 98 provides When the transducer is positioned so as to cause an' v switching arrangement.

arrangements maybe employed to provide a measure ofV signals L to `the"mixer circuit. When -the time tei at which the echo occurs is the same as the time tj at which the signals K areinitiated, a beatznote is' produced having a frequency which will be passed by the filter 56. The beat note is illustrated by the shaded area between the echo signals and the signals L of Fig. 10. This signal is detected by the detector 58, amplified by the video amplifier 60 and applied to an electrode in the cathoderay tube for intensifying the trace of the cathode-ray tube.

The angular position of the antennas and 92 is controlled by a motor 99. A potentiometer is coupled to the motor 99 to provide a voltage representative of thefangular position of the antennas. The output of the potentiometer 100 is applied to one set of the deflection.p1ates of the cathode-ray tube. The output of the variable voltage source 14 is coupled to the other set ofV deflection plates for the cathode-ray tube so as to provide a B-type scan, such as employed in conventional radar sets. l

It will be apparent that the sign-al frequencies which are passed by the band-pass or narrow-band filters S6 of the various embodiments of the invention may be adjustable if desired. This may be achieved by the use of tunable filters or by the use of multiple lters tuned to different frequencies which are selected by a suitable Such tunable or multiple filter Although linear frequency modulation has been illus-'l trated in the drawings for the variousV embodiments of the invention, frequency modulation having other waveshapes, such vas sinusoidal, may be employed provided the two frequency-modulated Awaves vary or sweep in substantially the same manner.

The techniques of the present invention have been described with reference to frequency modulation. These techniques are equally applicable to phase modulation arrangements, andthe term frequency modulation as used in the disclosure and claims is intended to include the special case where the frquency'modulation is of low order and thus c omes within the general usage of the term phase modulation. Also, the ter-m frequency modulation as used in the' disclosure and claims includes the special case where the frequency or phase modulation is zero.

The term propagation time as used in the specication and claims designates the time required for waves to travel from a source, such as a transducer to a target and then to a receiving location. Ordinarily the source and the receiving device are at the same location so that the waves travel from the source to the target from which they are reflected to the receiving device. The same transducer may be employed to transmit and receive the waves, or separate transducers may be employed. In the latter case the transducers may be located adjacent one another, or they may Ibe spaced apart.

I claim:

1. Apparatus for determining the propagation time of ultrasonic waves along a path, comprising an oscillator, means coupled to the oscillator for causing it to produce two Waves in time sequence having frequencies which vary periodically in substantially the same manner with the second wave starting at a later time and being offset in frequency from that of the first wave, a variablegain amplifier coupled to the output of the oscillator, a transducer coupled to the output of the amplifier for transmitting and receiving ultrasonic signals, means coupled to the amplifier for reducing the gain of the amplifier during the periods of time when the second waves are applied to it, means coupled to the transducer for mixing the received waves with the second waves which are then being produced to provide heterodyne signals, and means coupled to the mixing means for providing an indication '7 of heterodyne signals having va predetermined frequency'. 2. .Apparatus for 'determining the propagation time of ultrasonic waves saiong'a path, comprising a.tirnig pulse generator, 'a sweep generator coupled to the timing pulse.

pled to the output circuits of the sweep generator and the bi-stable circuit for providing an output signal of sawtooth Waveform wherein the successive voltage excursions start from and follow different voltage levels, 'oscillator means coupled to the output of the addition circuit yfor producing two waves :in .time Sequence having frequencies which vary periodically in substantially the same manner in synchronism with the voltage excursions which are produced by the addition circuit, with the second wave starting at a later time and being offset in frequency from that of the first wave, an amplifier coupled to the output of the oscillator means, a transducer coupled to the output of the 'amplifierv for transmitting andreceiving ultrasonic signals, a mixing `circuit coupled to the transducer for mixing the received waves with the waves which are then being produced to provide heterodyne signals, and means coupled -to the mixing means for providing an indi cation when the heterodyne signals have a predetermined frequency.

3. The apparatus of claim 2 wherein a hybrid network is connected between the transducer and the amplifier and the mixing circuit.;

4. The apparatus of claim 2 wheein the output of the bi-stable circuit ishcoupled to thejamplier to reduce the gain of the amplifier in synchonism with the occurrence of the second waves.

5. The apparatus of claimZ wherein the frequency of the vpulseswhich are produced by the timing ,pulse generator is varied periodically to vary the time at which the offset in frequency occurs.l Y

6. The apparatus of claim 2 wherein the lamount of offset in frequency between the two waves is varied periodically. l

7. The appratus of claim 2 wherein the location of the transducer is varied periodically to vary the length Aof the path along which the wavesare propagated.

r8. Apparatus for determining the propagationtime of. wavesalongfa path, comprising. means for producing, two waves1 iri time sequence which vary periodically in suh-A stantially the same manner with the second wave starting at later ,time than' the first wave and being offset in frequency from that of the first wave, means' for transmitting a-t least one of the wavesy along said path, means for receiving the waves which are reflected, means for causing ,the received waves to heterodyne with at least one of the waves' which are then being produced toprovide a signal having a certain heterodyne frequency when the received waves and the waves which are then being produced have a predetermined time relationship, said last-mentioned means including means for varying the path along which the waves are propagated to cause thev received waves and the waves whichV are thenV being produced to provide a signal having the certain heterodyne frequency, and means for providing an indication of the heterodyne signals having a selected frequency.

9. The method of determining the propagation time of waves along apath which comprises the steps of producing two waves in time sequence which vary periodically insubstantially the same lmanner with the second wave start-ing at a later time and being offset in frequency fromA that of the first wave, transmitting at least one of the waves `along. vsaid path, receiving the waves which are propagated-along the path, mixingl the received waves with at least one of the waves which are then being produced to provide heterodyne signals, varying the length of the path along which the waves are propagated to cause the lreceived wavesand the waves which are then being produced to provide a signal having a predetermined heterodyne frequency, and lproviding an indication of the heterodyne signals .having the predetermined frequency.

References Cited in the file of this patent UNITED STATES PATENTS 2,419,527 Bartelnk Apr. 29, 1947 2,424,833 Korman July 29, 1947 2,474,918 Slaymaker July 5, 1949 2,522,367 Guanella Sept. 12, 1950 2,562,977 Woodyard Aug. 7, 1951 2,640,106 `Wilson May 26, 1953 V2,659,878 Meeker Nov. 17, 1953 2,821,702 Russell Jan. 28, 1958 

